The present invention is within the field of aquaculture, in particular there is provided a method for producing prey organisms enriched in highly unsaturated fatty acids (HUFAs), particularly docosahexaenoic acid (DHA). Also provided are feed compositions based on such organisms.
The consumption of seafood species for which there is a high consumer demand such as salmon, trout, halibut and eel is increasing and due to this high demand and limited natural stocks, much effort is spent on developing cost effective aquacultural methods of farming such species. A particularly serious problem is to secure a high survival rate of the hatched larvae of the species being cultivated.
Expansion of the aquaculture industry requires that several problems be addressed, one of the most significant being the difficulty of supplying live prey organisms which provide a nutritionally adequate feed for the larvae. Larval fish in the wild consume a mixed population of phytoplankton prey organisms that provide a balanced nutrition. However, collecting phytoplankton in sufficient quantities to meet the demand in aquaculture is not feasible. As an alternative, selected species of prey organisms, in particular rotifers and Artemia species, are presently cultivated and used as feed.
Generally however, such artificially cultivated prey organisms, although they provide adequate amounts of protein and energy, have a lipid composition which is not adequate to cover the requirement for certain HUFAs, in particular DHA and EPA which are essential for the optimum survival, growth and development of larvae. Specifically, it has been shown that a high content of DHA is required and that the ratio between DHA and EPA in the prey organisms should be at least 1:1 and preferably at least 2:1. To provide prey organisms having such a composition in respect of HUFAs it is necessary to cultivate the organisms in the presence of enrichment compositions having a high content of DHA, preferably at least 20 wt % and a ratio of DHA to EPA exceeding the ratio aimed at in the prey organisms, such as at least 3:1 and preferably higher.
Currently, this problem is being addressed by cultivating the prey organisms in the presence of enrichment compositions permitting the organisms to be enriched in respect of these essential fatty acids. However, presently available commercial compositions for that purpose such as emulsion products sold under the tradename Selco (TM) do not meet the above requirements in that the DHA content is relatively low and/or the DHA:EPA ratio is not high enough. Using such compositions Artemia enrichment levels of 3-5% DHA of total lipids have been reported (McEvoy et al. Aquaculture 163 (1998) 237-250), and 12 to 15% survival rates of fish fed such Artemia (McEvoy et al. supra; Navarro et al. J. Fish Biol. 43 (1993) 503-515). In this context, survival rates are defined as survival percentage from the first feeding through metamorphosis. For cost-effective aquaculture production a larval survival rate of 50% and preferably higher should be obtained.
Other commercially available compositions for prey organism enrichment are products sold under the tradename Algamac (TM) containing up to 14 wt % of DHA, and tuna orbital oil (TOO) that contains up to 30 wt % of DHA.
WO 99/37166 discloses a method for the enrichment of live prey organisms with nutrients essential for fish larvae based on the use of dry soap powders of HUFAs obtained from the waste stream of marine algae oil extraction. The raw material for providing these powders has a content of phospholipids and it contains about 23 wt % of DHA, but apparently very little of other n-3 fatty acids. Artemia DHA enrichment levels of about 2.7% of dry weight are disclosed, but the use in aquaculture and efficacy with respect to predator larvae survival is not disclosed.
Another material intended for use in aquaculture is described in WO 99/06585. Examples disclose a DHA content of 24 wt %, but the phospholipid content is not disclosed. The material however, contains a high proportion of free fatty acids (about 32-37 wt %) and a high content of non-lipid material (about 39-44 wt %), which may reduce the lipid uptake efficiency of prey animals. A high content of free fatty acids is generally considered harmful for fish larvae and juveniles.
Neither of the two last-mentioned materials is fish-based and they lack many HUFAs found in fish, such as EPA and other n-3 fatty acids.
In a recent review by Sargent et al. (Aquaculture 179 (1999) 217-229) it is emphasized that in addition to the requirement in respect of HUFAs, fish larvae have a dietary requirement for phospholipids and it is stressed that the ideal diet for fish larvae is a diet having a composition similar to the yolk of the eggs. According to these authors fish egg yolk contains about 10 wt % (on a dry matter basis) phospholipids which contain about 17 wt % of DHA and about 9 wt % of EPA. These authors conclude in their review that a problem remains with respect to how to construct such a diet on a commercial scale from currently available materials.
It has now been found that it is possible to providexe2x80x94on a commercial scalexe2x80x94enriched aquacultural prey organisms having, in respect of HUFAs and phospholipids, a composition which is very close to that of fish egg yolk. By using the prey organisms of the invention it is possible to secure optimum survival, growth, pigmentation and morphogenesis of fish larvae such as halibut larvae. As demonstrated herein, the invention provides much higher survival rates during the larval stage and increased quality parameters than previously disclosed for fish such as Halibut, thus making aquacultural rearing of many high-demand fish species more economical and commercially viable.
In a first aspect, the invention provides a method of producing prey organisms for use in aquaculture, in particular for feeding larval fish, the method comprising cultivating said organisms during at least part of their life cycle in an aqueous medium comprising at least one lipid component having a DHA content of at least 30 wt %.
In a further aspect, the invention provides a composition for feeding fish in the larval and/or non-larval stage, the composition comprising the above mentioned prey organisms; said organisms having a content of DHA of at least 12 wt % of the total lipid content of the organisms.
In yet a further aspect, the invention provides a method of producing a composition having a water content that is less than 50 wt % including having a water content less than 10 wt % comprising producing live organisms according to the aforementioned method and at least partially separating the cultivated organisms from the aqueous medium.
In still a further aspect, the invention provides the use of the above compositions for feeding aquatic organisms at the larval and/or post-larval stage.